This invention relates generally to integratable amplifier circuits having variable gain control, and more particularly, to an integratable amplifier circuit susceptible to fabrication in a monolithic, bipolar integrated circuit form in which logarithmic gain control over a wide gain range may be effectively provided.
Logarithmic, digitally variable gain controlled amplifiers are known in the art. Such conventional amplifiers often employ linear digital to analog converters (DAC) to generate the logarithmic function by piece-wise linear approximation. When using the linear DAC the maximum attenuation obtainable is related to the number of bits employed in the digital input. For example, for a 4 bit linear DAC the maximum attenuation is 1/(2.sup.4)=.TM.=-24.1 dB. Accordingly, a 16 bit linear DAC is required to provide an attenuation of -96.3 dB=1/65,536 (2.sup.16 =65,536). Thus, an undesirably large number of bits is required to achieve a wide attenuation range employing the known linear DAC.
Logarithmic, digitally variable gain controlled amplifiers have been utilized for volume control over a large dynamic range. A paper in the IEEE Journal of Solid-State Circuits, Vol. SC-16, No. 6, December 1981, pages 682-689 entitled "A Volume and Tone Control IC for Hi-Fi Audio" describes a digital volume control circuit fabricated as a monolithic integrated circuit employing a metal-oxide semiconductor (MOS) process. Such MOS process is not compatible with other receiver and audio functions that have been integrated utilizing a bipolar process.